Andreas Albrecht, of University of California at Davis, talks about one of the current great mysteries of the universe - dark matter.View the Video

Most of the matter in the universe is dark. Without
dark matter, galaxies and stars would not have formed
and life would not exist. It holds the universe together.
What is it?

Although the existence of dark matter was suggested in the
1930s, only in the last 10 to 15 years have scientists made
substantial progress in understanding its properties,
mostly by establishing what it is not. Recent observations
of the effect of dark matter on the structure of the universe
have shown that it is unlike any form of matter that we have
discovered or measured in the laboratory. At the same time,
new theories have emerged that may tell us what dark matter
actually is. The theory of supersymmetry predicts new
families of particles interacting very weakly with ordinary
matter. The lightest supersymmetric particle could well be
the elusive dark matter particle. We need to study dark
matter directly by detecting relic dark matter particles in
an underground detector and by creating dark matter particles
at accelerators, where we can measure their properties and
understand how they fit into the cosmic picture.

From a vantage point a half-mile below ground, physicists of the Cryogenic Dark Matter Search have launched a quest to detect the dark matter that pervades the universe... read more

Most of the matter in the universe is dark. Early evidence for
dark matter came from the rotation curves of galaxies, which
showed that galaxies contain more mass than is contained in
the stars. More recently, direct evidence for dark matter has
come from the discovery and characterization of gravitational
lenses, regions of space where mass bends light. These
astronomical constraints do not directly distinguish between
nonbaryonic models for dark matter (WIMPs) and other possible
ideas involving more massive objects (MACHOs) such as Jupiter-sized
planets and mini-black holes. However experiments in the 1990s
established that MACHOs do not make an appreciable contribution
to the dark matter content of our galaxy.

The tightest constraints on the amount of dark matter in the
universe come from cosmological measurements. The frequency
and amplitude dependence of the fluctuations in the cosmic
microwave background (CMB) measured by WMAP (and in the future
by Planck) are sensitive to both the total matter density and
the baryon density. The baryon density is also constrained by the nucleosynthesis models of the early universe. All of these methods suggest that normal baryonic matter can only
account for a small fraction, about five percent, of the total
matter density.

MESSIER 31, a thin disk, is the largest galaxy in the neighborhood of the Milky Way. The distribution of mass in M31 and other galaxies led to the recognition that galaxies are filled with dark matter. Credit: Sloan Digital Sky SurveyView larger image

ACCURACY of WMAP, LHC and linear collider in determining the mass of the neutralino, a dark matter candidate, and so constraining its contribution to the energy density of the universe. Credit: Andreas BirkedalView larger image

What is dark matter? Particle physics models suggest that dark matter
is either axions (hypothetical new particles associated with QCD), or
WIMPs (hypothetical new particles with weak interactions and TeV-scale
masses, natural by-products of theories of supersymmetry or extra
dimensions). If dark matter particles are relics from the near-total
annihilation in the early universe, simple dimensional analysis
suggests that the particles originate from physics at the TeV scale.
The particle nature of dark matter can be verified by finding the rare
events they would produce in a sensitive underground dark matter detector
such as CDMS. Such experiments may see products of dark matter particles
in our galaxy. Annihilation of TeV-scale dark matter particles might be
detected as line radiation in high-energy gamma ray telescopes such as
GLAST and VERITAS, or possibly in astrophysical neutrino detectors such as ICE CUBE. Antiparticles produced in these annihilations might also be detectable by AMS. If
dark matter particles are much more massive, they might
produce signals in the ultra-high-energy cosmic rays.

However, to understand the true nature of dark matter particles,
particle physics experiments must produce them at accelerators
and study their quantum properties. Physicists need to discover
how they fit into a coherent picture of the universe. Suppose
experimenters detect WIMPs streaming through an underground
detector. What are they? Are they the lightest supersymmetric
particle? The lightest particle moving in extra dimensions? Or
are they something else?

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Searches for candidate dark matter particles are underway at
present-day colliders. If these particles have masses at the TeV
scale, they will surely be discovered at the LHC. However,
verifying that these new particles are indeed related to dark
matter will require the Linear Collider to characterize their
properties. The Linear Collider can measure their mass, spin
and parity with precision. These results will permit calculation
of the present-day cosmic abundance of dark matter and comparison
to cosmological observations. If the values agree, it will be a
great triumph for both particle physics and cosmology and will
extend the understanding of the evolution of the universe back to
10^-10 seconds after the big bang.